Le 10 juin 09 à 12:20, Álvaro García Pérez a écrit :
I agree that there are some issues about the cyclic dependencies with the observer pattern,
Note it is not a problem about _cyclic_ dependencies, the dependencies I have shown is a directed acyclic graph. Cyclic dependencies are yet another issue you can solve for example with fixed points and infinitesimal delays but it's a different problem.
but I think that considering it a glitch or not is a matter of phylosophical discussion.
The semantics you really want is no glitches (= instantaneous propagation times aka synchrony hypothesis). Because you want to think about your values as being for all t : a(t) = b(t) + c(t) and you can't do that if you allow the glitches to occur. You do the same kind of reasoning in electronics when you introduce latches and clocks.
In Daniel's example you will end with the proper value in "a" (even if you have an inproper value for some negligible instants). You only may consider this a problem if your system has poles near those values, which can turn it into a diverging system, and if the updating period is big enough to trigger those diverging effects. For almost any non real-time-control system this is not alike to happen.
You may think it's marginal but it's not: you get into problems as soon as your update functions may performs some kind of side effects (e.g. write something to a file). A friend of mine ran exactly into the problem I described with the observer pattern in Cocoa, all the glitches triggered expensive and unused graphical updates and were ruining the interactive experience with the system. Eventually he had to side-step the observation mechanism for certain parts of his system.
And of course, you can implement your own Observer patern imposing some policies in the update ordering, which can help to avoid the glitches.
Yes, you'll end up implementing an frp system. You'll see this will be quite different from an observer pattern implementation because you need more contextual information about the observers to perform the update of the dependents under a synchrony hypothesis. Best, Daniel